# 1.7vdc to 3vdc in, 10.3 to 17 out

#### Josephny

Joined Nov 2, 2018
8
I don't have much experience or knowledge, so please excuse the basic question.

I have an sensor that outputs between 1.7vdc and 3vdc and I need to convert it, in a linear way, to 10.3 to 17vdc.

Is there an easy circuit or IC that I can use for this?

For anyone interested, the sensor reads oxygen level in car exhaust which has a linear relationship to air-fuel mixture in a carb. For example, 2.5vdc equates to an air-fuel ratio of 14.7:1, which is perfect at idle or cruise.

Thank you!

#### Papabravo

Joined Feb 24, 2006
14,461
Use a single supply operational amplifier in a non-inverting configuration with a voltage gain of 14.24 dB and an offset of 8.6 Volts. The supply voltage should be 18-20V DC

#### Josephny

Joined Nov 2, 2018
8
Thanks very much!

First thing I notice is that I don't have 18v available as this is a 12v automotive environment.

Then, I've never worked with a supply op amp, so I don't know where to start -- or if it's even possible given the 12v limit.

Joined Mar 10, 2018
4,057

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#### ebp

Joined Feb 8, 2018
2,332
Why do you want a maximum output voltage exceeding the available supply voltage for an automotive application? It seems rather odd. I sort of get the impression that you want a voltage numerically equal to the air-fuel ratio, perhaps to display on some sort of meter.

How much current does the output have to supply? If it is more than a few milliamperes, the circuit required will be more complex.

If that really is what you require, you can power the amplifier circuit with a DC-DC converter. As long as the current output is low, a small, easy to use inexpensive (few dollars) module will do.

If you do want this for display on a meter, there may be ways to change the scaling of the meter. It might be quite easy to change the meter to display 14.7 when the actual input voltage is 1.47 or 3 (as you get directly from the sensor) or ...

#### Papabravo

Joined Feb 24, 2006
14,461
Thanks very much!

First thing I notice is that I don't have 18v available as this is a 12v automotive environment.

Then, I've never worked with a supply op amp, so I don't know where to start -- or if it's even possible given the 12v limit.
With respect to power supplies, a "single supply op amp" uses one power supply voltage. A "bipolar supply op amp" use two supply voltages, one positive and one negative.

There is no such thing as a s"supply op amp".

With a DC-DC converter or a charge pump you can create voltages other than the ones you have available from your primary power source. A limited knowledge of circuits is going to make those techniques difficult for you to apply in order to meet your requirements which were not fully defined in the original post. It sounds like you should stick to the original voltage output from the sensors and use a microprocessor with a A/D converter to get your display values.

#### Josephny

Joined Nov 2, 2018
8
Thank you all so much for the suggestions and explanation.

Let me explain more about the project:

I have a 1966 Mustang, 200 cubic inch inline 6 cylinder engine with a single barrel carburetor. At idle (or cruise) speed, the air to fuel ratio, controlled by the carb, should be 14.7. This will vary across the speed/power curve of the engine -- leaner or richer as needed. This tuning is very important to how well the car performs: smooth engine running, power, acceleration, fuel efficiency, etc.

There is a proportional relationship between the AF ratio at the carb (14.7 ideally at idle), and the amount of oxygen in the exhaust. Many modern cars have O2 sensors built in to their exhaust and the data is fed back to a circuit that adjusts the AF mixture as needed.

This wasn't the case in 1966 (;-)

So, I've welded a threaded nut into the exhaust and I have a Bosch Wideband sensor that screws into it. This sensor will produce between 0 and 5vdc proportionally according to the O2 level in the exhaust. For example, 2.5v indicates an AF ration of 14.7.

So, I have a little LED voltmeter that I'd like to mount on the dash. I can certainly have it display 2.5 and convert in my head to 14.7, but it sure would be nice to have the meter display 14.7 when 2.5 comes out of the sensor.

So that's how I come to be here.

Thanks!

#### Alec_t

Joined Sep 17, 2013
11,665
Can you post a link to your voltmeter datasheet?

Edit: If you could just mentally shift the decimal point to the right, then you could easily convert the 1.7-3V range to a 1.03-1.7V range and avoid needing a circuit supply voltage greater then 17V.

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#### Josephny

Joined Nov 2, 2018
8
Getting around the supply voltage above 12 by mentally shifting the decimal is a great idea and I would have no problem looking at, for example, 1.47 and seeing it as 14.7.

But, how would I map that?

This is the mapping (it's a straight line):

VOLTAGE AF RATIO
0.00 9.00
0.50 10.00
1.00 11.00
1.50 12.00
2.00 13.00
2.50 14.00
2.85 14.70
3.00 15.00
3.50 16.00
4.00 17.00
4.50 18.00
5.00 19.00

The voltmeter is just off Amazon:

Product features
• 3 Wires. Able to measure the voltage that is below 3V. Measurement Accuracy: 1%
• Display: Three 0.36" digital LED Tube. Display Color: Red. Measure Voltage: DC 0~30V. Working Voltage: DC 3V-30V
• Reverse Polarity Protection: The meter won't burn out if reverse polarity is applied, but the screen will display nothing
• Can be calibrated. The screw on the back is used to calibrate the reading
• Operating Temperature: 14 ~ 149 °F(-10 ~ 65 °C)
Product description
Features:
Cutting size: 23mm x 14mm/0.9" x 0.55", Thickness: 10mm/0.39"
Mounting Hole Center Distance: 28mm
Measure Voltage: DC 0~30V
Working Voltage: DC 3V-30V
Measurement Accuracy: 1% (+ / - 2 digit)
Display: Three 0.36" digital LED Tube
Display Color: Red
Refresh Rate: about 500mS / times
Operating Temperature: 14 ~ 149 °F(-10 ~ 65 °C)
Reverse protection: Yes
When measured voltage is below 10V: the voltmeter displays two decimal digits, for example, 5.32V
When measured voltage is larger than 10V, the voltmeter displays one decimal digit, for example, 10.5V

Wiring:
Red wire: Power supply +
Black wireower supply -, Measured Voltage -
White wire: Measured Voltage +

Package Includes:
5x DC 0-30V Voltmeter

#### Alec_t

Joined Sep 17, 2013
11,665
But, how would I map that?
It can be done using an op-amp to scale and offset the sensor voltage. Fortunately your voltmeter is a 3-wire type which can read input voltages below its supply voltage.

#### crutschow

Joined Mar 14, 2008
25,462
Here's the LTspice simulation of a circuit that converts 0-5V to 0.9V-1.9V.
It uses two single-supply opamps in one IC package, and a TL431 2.5V reference to provide an accurate offset voltage.

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Joined Mar 10, 2018
4,057

I did not pick the best OpAmp, a RRIO will lower the required 24 V supply to
~ 18V.

Regards, Dana.

#### Josephny

Joined Nov 2, 2018
8
View attachment 163071

I did not pick the best OpAmp, a RRIO will lower the required 24 V supply to
~ 18V.

Regards, Dana.
Wow! That is so cool!

I only have 12v to work with (which will vary from from just under 12 to ~14) and the Vin range is 0-5.

Thank you!

Joined Mar 10, 2018
4,057
If you just have 12V then you will need a boost converter to get to 18.
Something like the one in post #7 in this thread.

Note you also need a Vref, could be a zener (a low precision solution)
or a fixed 5V one like an LM336, can be had with initial tolerance
of 1%. no need to adjust.

Lastly if you decide you need a different range for the conversion just go to
the link and plug in the new range, the calculator will give you the new R
values needed.

http://earmark.net/gesr/opamp/gain_offset.htm

You state the Vin range is 0 -5 V ? Your initial post was 1.7 - 3. Is
the range you want to control the 10.3 - 17, but input also exceeds
the 1.7 - 3 or do you want 0 -5 to control 10.3 - 17 ?

Regards, Dana.

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#### Josephny

Joined Nov 2, 2018
8

So the TL072 circuit above, with R values for 0-5Vin to a 9-19Vout.

An LM336 (regulator) for 5Vref.

Sensor out to V2.

But I'm confused about which connection to the meter/display input and which get the 18v from the "DC-DC Boost Step up Volt Converter Power Supply" on ebay?

I'm also confused about the pinouts of the TL072; pinouts are 1-8 and I don't know how to convert those to the schematic.

Finally, I downloaded ADIsimPE (so cool!) -- can you make this schematic available so I can import?

Thank you so much!

Joined Mar 10, 2018
4,057

So the TL072 circuit above, with R values for 0-5Vin to a 9-19Vout.
So use the link I gave you to generate the R values, just plugin your Vin and Vout range,
it will do the rest. The invert, non-invert pins go to the schematic respective resistor
junctions. The -V Opamp pin goes to ground, same for boost inverter ground output
pin.

The boost converter outut connects to the OpAmp +V pin.

The OpAmp output pin goes to the meter. What is your meter, will it handle 20V input ?

When you set the boost converter V it should be 2 volts greater than maximum
output you want.

Regards, Dana.

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#### Josephny

Joined Nov 2, 2018
8
ADIsimPE is quite a powerful tool -- a whole new learning curve to tackle.

I've been trying to work with the earmark resistor valve calculator as ADIsimPE, but I'm new to all of this, so I'm having a difficult time.

Plugging in Vmin=0, Vmax=5, I get:

But when I tried to run the simulator it in ADIsim, I received a "one or more pin names on instance X1 are illegal".

When I selected "Choose Analysis and put a start and stop value of 0 and 5 and then put a probe at the output, I got:

And this is my modified schematic:

I'm sure I've got some things wrong and am missing some components or calculations.

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Joined Mar 10, 2018
4,057
Yes you have to change the resistors to the new values on the ADSIsim schematic.

V1 should be a fixed power supply Voltage, ~ 20 - 24.

The LM317 gnd looks like it is nopt placed correctly on the LM317 symbol pin
in ADIsim. And how do you set its output V ?

And you sure the 12V is connected to LM317 input pin ? That symbol
is mirrored on schematic as you show it, and that LM317 output pin is connected
to Vref.

Regards, Dana.

#### Josephny

Joined Nov 2, 2018
8
It's clear I'm in over my head.

I did change the resistor valves in ADIsim, but couldn't get the output graph to change.

I don't know how to show or determine which pin is which on the LM317.

I was assuming the DC-DC Step up would provide a fixed output, but I see now that it will vary with the input (which does vary in an auto environment). I supposed I could use the Step up to take the auto voltage up to 24 and then regulate down to 18.